There are many correct answers to this question, as a proper noun could be characterized as many things. A proper noun can also be referred to as a proper name. As the name suggests, a proper noun is a noun. Its purpose is to represent a unique thing.
Examples include a model of a car, such as Toyota or Nissan, or a name, such as John Doe. The proper noun is unique to the thing it is representing. One can easily identify a proper noun in a sentence because they are the only nouns in which the first letter must be capitalized, regardless of where in the sentence it falls.